Active Galactic Nuclei as Factories for Eccentric Black Hole Mergers

Abstract

Black hole mergers detected by LIGO and Virgo continue delivering transformational discoveries. The most recent example is the merger GW190521, which is the first detected with component masses exceeding the limit predicted by stellar models, and the first with non-zero orbital eccentricity. The large masses can be explained by build up through successive mergers, which has been suggested to occur efficiently in the gas disks of active galactic nuclei (AGN). The eccentricity, however, is a major puzzle. Here we show that AGN-disk environments naturally lead to a very high fraction of highly eccentric mergers, if interactions between binaries and singles are frequent, and the interactions are constrained to a plane representing the AGN-disk. By deriving a statistical solution to the chaotic 3-body problem with the inclusion of General Relativistic corrections, we find in our fiducial AGN-disk model that up to 70 percent of all black hole mergers could appear with an eccentricity >0.1 in LIGO/Virgo. Besides representing the most effective mechanism for producing eccentric mergers presented to date, our results have also profound implications for the origin of GW190521, and open up new lines of research on black hole scatterings in disk environments with far-reaching implications for gravitational wave astrophysics.

Detecting gravitational lensing in hierarchical triples in galactic nuclei with space-borne gravitational-wave observatories

“…However, eccentricities in both orbits may be expected, especially if the inner binary is formed via dynamical channels (see, e.g., Refs. [22][23][24][25][26][27][28][29][30][31]). The qualitative effects of eccentricity have been argued in Ref.…”

“…The inner binary also needs to be able to survive evaporation due to dynamical interactions with environmental stars on a time scale of typically a few Myr [23,83]. These conditions could be satisfied by both the gaseous channel [15] and the dynamical channels [31], though the inner binary produced by some dynamical channels may have too high an eccentricity. Such an inner binary may have too short a GW decay timescale and merge before it orbits the SMBH by a complete cycle, disfavoring the detectability of both lensing and precession.…”

“…In addition to shrinking the orbit of the BBH itself, gas can also help the center of mass of the BBH to migrate in the AGN disk and thus alter its orbit around the SMBH [20,21]. Even without gas, dynamical interactions of a variety of flavors may also help the formation of compact BBHs [22][23][24][25][26][27][28][29][30][31], providing yet another channel for the formation of "BBH þ SMBH" triple systems.…”

Implications of Eccentric Observations on Binary Black Hole Formation Channels

“…For example, a BBH composed of two 20 M black holes (BHs) at an initial orbital separation of 1 R and initial eccentricity of 0.9 (0.99) will have an eccentricity at a GW frequency of 10 Hz of 0.001 (0.05) and merge in 32 yr (6 day). The only means of producing measurably eccentric BBH mergers in the LIGO-Virgo band that does not require a high degree of fine-tuning is through strong gravitational encounters in dynamical environments (e.g., O'Leary et al 2009;Kocsis & Levin 2012;Samsing et al 2014;Rodriguez et al 2018b;Samsing et al 2018;Samsing 2018;Takatsy et al 2019;Zevin et al 2019;Gondán & Kocsis 2020;Gröbner et al 2020;Samsing et al 2020b;Tagawa et al 2021) or through channels that can pump eccentricity into inspiraling binaries, such as the secular evolution of hierarchical systems (e.g., Antonini & Perets 2012;Antognini et al 2014;Silsbee & Tremaine 2017;Antonini et al 2017;Rodriguez & Antonini 2018;Fragione & Bromberg 2019;Fragione & Kocsis 2019;Liu & Lai 2019).…”

Anomalies in the gravitational recoil of eccentric black-hole mergers with unequal mass ratios

“…However, viable dynamical formation channels of stellar-origin BH binaries could result in a non-negligible population of merging BHs that still retain moderate eccentricities at frequencies relevant for ground-based GW detection (see, e.g., Refs. [43][44][45][46][47][48]). Furthermore, the presence of astrophysical media such as accretion disks may increase the eccentricity during the inspiral [49].…”